1. Field of the Invention
The present invention relates to optical data communications networks, in particular but not exclusively to a TPON (telecommunications over passive optical networks) network.
2. Description of the Prior Art
In a TPON network, a network head-end station (for example in a telephone exchange) is linked to plurality of remote terminations (for example street distribution points) by a passive optical splitting network (PON). There may be separate PONs for the downstream (head-end to terminations) and upstream (terminations to head-end) data directions.
In the TPON network, according to a bit transport system (BTS) proposed by British Telecommunications PLC, in the downstream direction data is broadcast by the head-end station to all the terminations in the network in a time division multiple access (TDMA) multiplexed frame. In the upstream direction, each termination transmits a data pulse in a predetermined time slot so that the data pulses reaching the head-end station from the different terminations are interleaved to form an upstream TDMA frame having a predetermined format.
The link loss (optical attenuation), between a termination and the head-end station, may vary considerably from one termination to another in dependence upon the distance of the termination concerned from the head-end station and upon the number of optical splitters through which the optical signal from the termination must pass to reach the head-end station. This means that, assuming all of the terminations transmit their data pulses at the same optical power level, the interleaved upstream data pulses reaching the head-end station will vary significantly from one another in optical power.
Heretofore it has been thought that it would not be possible for the head-end station optical receiver to cope with such wide variations in received optical power levels, and accordingly, in a trial system, the optical output power level of each termination was controlled individually, for example in dependence upon control data included in the downstream TDMA frame broadcast to the terminations by the head-end station, so as to achieve more uniformity in the received optical power levels at the head-end station.
Such individual control of the terminations, however, is undesirable in that it increases the complexity, and hence the cost, of the opto-electronic equipment at each termination. The cost of the terminations is often a critical element in the feasibility of any network. Moreover, the need to transmit control data in the downstream TDMA frame, at least on initialisation of a termination and probably at intervals during use thereof to overcome drift, may result in slow initialisation and can generally reduce the time available for transmission of data.